What is the role of ultrasound in the first trimester for a pregnant female, particularly for assessing fetal development and potential complications?

First Trimester Ultrasound: Essential Role in Pregnancy Assessment

First trimester ultrasound is a critical diagnostic tool that should be offered to all pregnant women between 11-14 weeks for accurate pregnancy dating, aneuploidy screening via nuchal translucency measurement, early detection of structural anomalies (particularly cardiac defects), and assessment of pregnancy viability and multiplicity. 1

Primary Indications and Timing

Standard First Trimester Scan (11-14 Weeks)

• Nuchal translucency (NT) measurement is the cornerstone screening tool for chromosomal abnormalities, particularly Down syndrome, with approximately 70% detection rate at 5% false-positive rate when performed between 11-14 weeks 1

• The scan should assess fetal viability, number of fetuses, accurate gestational age via crown-rump length (CRL), and NT measurement 2

• First trimester screening combining NT measurement with serum markers (PAPP-A and hCG) is an acceptable, cost-effective approach for Down syndrome risk assessment when women present before 14 weeks 1

• An increased NT measurement (>95th percentile) is significantly associated with not only aneuploidy but also congenital heart defects, diaphragmatic hernias, skeletal dysplasias, and various genetic syndromes 1

Early Anatomic Assessment (12-13+6 Weeks)

• Detailed first trimester anatomic evaluation can detect approximately 27.3% of structural abnormalities, particularly defects of the cranial vault, midline brain, and abdominal wall 2

• Early detection of cardiac anomalies is a major advantage of first trimester ultrasound, as substantial evidence shows congenital heart defects may present with increased NT 1

• First trimester fetal echocardiography between 12-13+6 weeks is appropriate for high-risk cases including previous child with congenital anomaly, maternal diabetes, IVF pregnancy, or multiple gestations 2

Multiple Gestations: Critical Early Assessment

Chorionicity Determination

• The 11-14 week scan MUST include assessment of chorionicity and amnionicity in multiple pregnancies, as this fundamentally determines surveillance intensity and pregnancy risk 1, 2

• Monochorionic (MC) twins account for only 20% of twin pregnancies but 30% of all complications, with overall mortality rate of approximately 10% primarily from twin-to-twin transfusion syndrome (TTTS) 1

• Discrepant crown-rump length measurements between twins may be an early marker for TTTS and warrants intensified surveillance 2

Surveillance Schedule for Multiples

• Multiple gestations require at minimum: first trimester dating scan, NT scan at 11-14 weeks, anatomy scan at 18-22 weeks, and one or more third trimester growth scans 1

• Monochorionic diamniotic pregnancies require surveillance beginning at 16 weeks with weekly to biweekly monitoring for TTTS, including fetal biometry every 2-3 weeks and weekly amniotic fluid assessment 3

Screening vs. Diagnostic Testing: Critical Distinction

Non-Invasive Screening

• First trimester combined screening (NT + PAPP-A + hCG) provides risk assessment, NOT diagnosis 1

• Cell-free DNA testing (NIPT) is a screening test that requires confirmatory invasive testing for all positive results, as false positives occur from placental mosaicism, maternal contribution, vanishing twin, or maternal malignancy 4

Definitive Diagnosis

• All women regardless of age should be offered invasive diagnostic testing (CVS or amniocentesis) to definitively identify major fetal aneuploidies after appropriate counseling about risks and benefits 1

• Chorionic villus sampling (CVS) can be performed at 10-13 weeks with pregnancy loss risk of approximately 1 in 300-600 procedures 1, 4

• Amniocentesis is performed from 15 weeks onward and is the most reliable diagnostic method, particularly when mosaicism is suspected 4

• CVS results showing mosaicism are NOT confirmatory due to confined placental mosaicism risk and require follow-up amniocentesis 4

Additional Clinical Applications

Pregnancy Localization and Viability

• Transvaginal ultrasound is the modality of choice for establishing intrauterine pregnancy location, evaluating viability, and diagnosing ectopic pregnancy in early first trimester 5, 6

• Ultrasound provides accurate gestational age assessment, which is critical for timing subsequent screening and interventions 7, 8

Optional Advanced Markers

• Nasal bone assessment may be incorporated into first trimester screening but should be limited to clinicians with specific training and ongoing quality assurance participation, as absence is associated with Down syndrome but standardization is controversial 1

Common Pitfalls to Avoid

• Do NOT perform Doppler studies (umbilical artery, middle cerebral artery) at 11-14 weeks as they have no established role at this gestational age 2

• Biophysical profile has no role in first trimester and should not be performed 2

• Do NOT consider CVS results showing mosaicism as diagnostic—these require amniocentesis confirmation due to confined placental mosaicism 4

• Avoid using MRI with gadolinium contrast in first trimester, as gadolinium crosses the placental barrier and long-term fetal risks are unknown; MRI without contrast may be considered after first trimester for complex diagnoses 1

• Women who decline aneuploidy information should not be required to undergo screening or diagnostic testing after appropriate documentation 1

Patient Autonomy Considerations

• Women have the right to decline all aneuploidy screening and diagnostic testing after appropriate counseling and documentation 1

• Screening provides an option for women who want to refine their risk before deciding on invasive testing, balancing the desire for information against procedural risks 1